1. Field of the Invention
The present invention relates to a catalyst for purifying an exhaust gas discharged from an internal combustion engine of a motor vehicle or the like, and an exhaust gas purifying method using such catalyst. More particularly, it relates to the catalyst and the purifying method capable of efficiently reducing and purifying nitrogen oxides (NOx) contained in the exhaust gas. The exhaust gas contains excess oxygen of amount more than that required for complete oxidation of reduction components such as carbon monoxide (CO), hydrogen (H2) or hydrocarbon (HC).
2. Description of the Related Art
Conventionally, as the catalyst for purifying the exhaust gas of the motor vehicle or the like, a three way catalyst to purify the exhaust gas by oxidizing CO and HC and reducing NOx in the exhaust gas simultaneously in a theoretical air-fuel ratio (stoichmetric) has been used. A well known example of the three way catalyst is composed of a heat-resistant substrate made of cordierite or the like, and a porous support layer made of xcex3-alumina and formed thereon. The catalyst nobel metals such as platinum (Pt), rhodium (Rh) or the like are loaded on the porous support layer.
On the other hand, recently, in order to protect an earth surrounding environment, carbon dioxide (CO2) contained in the exhaust gas of the internal combustion engine of the motor vehicle or the like has been required to be reduced. As the system capable of meeting this requirement, xe2x80x9clean burnxe2x80x9d system in which the fuel is burnt leanly in the excess oxygen contained atmosphere has attracted much attention. With the lean burn system, decreasing the fuel consumption can restrain emission of CO2 generated by burning or combusting the fuel.
On the contrary, the conventional three way catalyst oxidizes or reduces CO, HC or NOx simultaneously to purify the exhaust gas when the air-fuel ratio is in the theoretical air-fuel ratio (stoichimetric), but it does not exhibit sufficient purifying performance for NOx removal by reduction in the excess oxygen contained atmosphere of the exhaust gas in the lean burn system. Under these circumstances, development of both the catalyst and the purifying system capable of efficiently purifying NOx even in the excess oxygen contained atmosphere has been demanded.
The present Applicant have proposed the catalysts for purifying the exhaust gas, in which Pt and alkaline-earth metal such as Barium (Ba) are loaded on a porous support made of alumina or the like (Japanese Patent Laid-open No. 5-317625).
With this catalyst, by controlling the air-fuel ratio from the lean side to the stoichimetric or rich side like a pulse (rich pulse), NOx is occluded in the alkaline-earth metal (NOx occluding material) when the air-fuel ratio is lean, and reacts with reduction components such as HC or CO contained in the exhaust gas to be purified when the air-fuel ratio is stoichimetric or rich. Thus, even in the lean burn system, the NOx can be purified efficiently.
It has been known that the purifying reaction of the NOx by the above catalyst for purifying exhaust gas is comprised of a first step for oxidizing NO contained in the exhaust gas to NOx, a second step for occluding NOx in the NOx-occluding material, and a third step for reducing NOx emitted from the NOx-occluding material -by the HC and CO contained in the exhaust gas on the catalyst.
However, the conventional catalyst for purifying exhaust gas has been suffered from disadvantage that in the lean atmosphere particle growth occurs in Pt to decrease the number of catalyst active points thereof, which decreases the reacting performance in the above first step and the third step.
On the other hand, Rh has been known as the catalyst noble metal hardly suffering from such particle growth in the lean atmosphere, but the oxidizing ability of Rh is inferior to that of Pt. In view of this, using both of Pt and Rh as the catalyst has been studied.
However, when both of Pt and Rh are used, the disadvantage of Rh hindering the oxidizing ability of Pt has clarified. That is, as the added amount of Rh increases, Rh decreases the reacting performance of Pt oxidizing NO to NOx in the first step, and it decreases the NOx occluding ability of the NOx occluding material in the second step. Further, due to bad sympathy of Rh with the NOx occluding material, the co-existence of Rh and the NOx occluding material prevents them from exhibiting the characteristics thereof.
In addition, when the NOx occluding material reacts with sulfur oxides (SOx ) such as sulfur oxide (SO2) contained in the exhaust gas to have created the salt, it can not exhibit the NOx occluding function any more. Further, since the formed sulfate or sulfite is not dissolved easily, the NOx occluding material can hardly exhibit the natural NOx occluding function thereof. This is called xe2x80x9csulfur poisoningxe2x80x9d which means poisoning of the NOx occluding material by sulfur.
Japanese Patent Laid-open No. 10-356 discloses a catalyst for purifying exhaust gas produced by mixing first powders in which Rh is loaded on alumina or zirconia, and second powders in which Pt and the NOx occluding material are loaded on alumina In this catalyst for purifying exhaust gas, due to the separated loading of Pt from Rh, Pt is restrained from decrease of the oxidizing ability by Rh. Also, Rh and the NOx occluding material being separately loaded do not suffer from the bad mutual sympathy, and can exhibit the characteristics thereof sufficiently.
Furthermore, the separately loaded Rh assists so that HC and H2O both contained in the exhaust gas create the hydrogen having large reducing force (steam reforming reaction) which contributes reduction of the NOx and separation of the SOx from the sulfate or the sulfite in the NOx occluding material. In this way, the NOx reducing amount in the rich pulse is increased, and the sulfur poisoning of the NOx occluding material is decreased remarkably.
By the way, as the support for loading Rh thereon, using zirconia capable of increasing the steam reforming reaction activity of Rh is preferable. However, the zirconia has smaller heat resistant performance than that of the alumina often used as the support for the noble metal, whereby the specific surface of the support is decreased by heat when the catalyst is used for purifying exhaust gas. As a result, the dispersing performance of the loaded Rh decreases, which results in decrease of the purifying ability thereof.
In addition, due to insufficient increasing operation of the steam reforming reaction activity of Rh by the zirconia, development of the support capable of further increasing the steam reforming reaction activity of Rh has been expected.
The present invention has been made in view of the above circumstances, and has a first object to maintain the oxidizing ability of Pt high by restraining decrease of the oxidizing ability of Pt by Rh.
A second object of the present invention is to cause Rh to exhibit the steam reforming reacting activity sufficiently by using the zirconia as the support.
Further object of the present invention is to restrain the poisoning of the NOx occluding material by the sulfur.
A catalyst for purifying exhaust gas recited in the claim 1 and can overcome the above subject matter is prepared by mixing first powders in which at least platinum (Pt) is loaded on a first support made of porous particles and second powders in which at least Rh is loaded on a second support, and loading NOx occluding material at least on the first support. The catalyst for purifying exhaust gas is featured by that the second support is made of alkaline-earth metal contained zirconia having contained alkaline-earth metal by amount not more than 11.35 mole %.
The catalyst for purifying exhaust gas recited in the claim 3 is featured by that, in the catalyst for purifying exhaust gas recited in the claim 1, the second support is made of the zirconia having contained at least one of elements selected from barium and calcium.
An exhaust gas purifying method recited in the claim 4 is featured by that the exhaust gas discharged from an internal combustion engine in which the fuel is combusted in condition where mixed gas of rich atmosphere containing excessive fuel is supplied to mixed gas of lean atmosphere containing excessive oxygen like a pulse, is purified by a catalyst for purifying exhaust gas. Here, the catalyst for purifying exhaust gas is produced by mixing first powders in which at least Pt is loaded on a first support made of porous particles and second powders in which at least Rh is loaded on a second support made of alkaline-earth metal contained zirconia having contained alkaline-earth metal by amount not more than 11.35 mole %, and loading NOx occluding material at least on the first support.
The exhaust gas purifying method recited in the claim 6 is featured by that, in the exhaust gas purifying method recited in the claim 4, the second support is made of the zirconia having contained at least one of elements selected from barium and calcium.
The catalyst for purifying exhaust gas recited in the claim 7 is prepared by mixing first powders in which at least platinum (Pt) is loaded on a first support made of porous particles and second powders in which at least Rh is loaded on a second support, and loading NOx occluding material at least on the first support. The catalyst is featured by that the second support is made of rare-earth element contained zirconia having contained rare-earth element by amount not more than 11.0 mole %.
The catalyst for purifying exhaust gas recited in the claim 8 is featured by that, in the catalyst for purifying exhaust gas recited in the claim 7, the second support is zirconia having contained lanthanum.
The exhaust gas purifying method recited in the claim 9 for purifying exhaust gas discharged from an internal combustion engine in which fuel is combusted in condition where mixed gas of rich atmosphere containing excessive fuel is supplied to mixed gas of lean atmosphere containing excessive oxygen like a pulse, by a catalyst for purifying exhaust gas prepared by mixing first powders in which at least Pt is loaded on a first support made of porous particles and second powders in which at least Rh is loaded on a second support made of rare-earth element contained zirconia having contained rare-earth element by amount not more than 11.0 mole %, and loading NOx occluding material at least on the first support.
The exhaust gas purifying method recited in the claim 10 is featured by that, in the exhaust gas purifying method recited in the claim 9, the second support is zirconia having contained lanthanum.